This invention relates to imprints printed by a print head and to alignment of imprints printed in successive swathes by the print head one with another.
Digital print heads are known in which a plurality of print elements are disposed in a line and are actuated selectively in a succession of print cycles. Selective actuation of the print elements in a print cycle effects printing of dots in corresponding selected locations in a column on a print receiving medium. The print heads may be ink jet print heads in which droplets of ink are ejected selectively from a row of nozzles onto the print receiving medium.
The print head is transported relative to the print receiving medium in a direction perpendicular to the line of print elements so that the columns in which printing is effected in successive printing cycles are spaced in a direction perpendicular to the columns. One use for such print heads is in printers of postage meters where the printer is required to print postage indicia on mail items. In commonly available ink jet print heads, the extent of the row of nozzles is too short to print a postage indicium in a single pass of the print head. Accordingly it is proposed to cause the print head to traverse a print field of the mail item in a first direction in a first pass and to traverse the print field in an opposite direction in a second pass, the print head being displaced by a distance approximately equal to the length of the row of nozzles after the first pass so that in the first pass a first swathe of the indicium is printed and in the second pass a second swathe of the indicium, adjacent to the first part, is printed. -In order that the completed indicium is correctly printed without an interface between the first and second swathes being visible, it is necessary to ensure that the second swathe of the indicium is printed in alignment, in the direction of traverse of the print head, with the first swathe.
According to one aspect of the invention a method of alignment of a first imprint printed in a first traverse in a first direction along a first track by a print head having a plurality of selectively energisable printing elements disposed in a line with a second imprint printed in a second traverse in a second direction opposite to said first direction along a second track displaced relative to said first track includes the steps of generating signals corresponding to increments of traverse of the print head; during the first traverse of the print head initiating printing of a first imprint; upon completion of printing said first imprint initiating incrementing a counter from a first count by said signals; terminating incrementing the counter in response to sensing of the print head passing a reference position; and during the second traverse of the print head initiating decrementing of the counter by said signals in response to sensing of the print head passing the reference position; and in response to decrementing the counter to a second predetermined count initiating printing of the second imprint.
The first and second counts may be equal or the second count may be offset from the first count by a predetermined correction count.
According to a second aspect of the invention printing apparatus includes a print head having a plurality of selectively energisable printing elements disposed in a line; drive means to traverse the print head in a first traverse in a first direction along a first track and to traverse the print head in a second direction opposite to said first direction along a second track, said second track being displaced relative to the first track in a third direction transverse to said first and second directions; print head control means operable to selectively energise said printing elements during the first traverse of the print head to print a first imprint and to selectively energise said printing elements during the second traverse of the print head to print a second imprint adjacent said first imprint; a counter; the control means being operative during said first traverse to initiate stepping of the counter in correspondence with increments of traverse of the print head in the first direction upon completion of printing the first imprint and operative in response to sensing traversing of the print head past a reference location in the first traverse to terminate stepping of said counter and said control means being operative during said second traverse in response to sensing traversing of the print head past the reference location in the second traverse to initiate stepping of the counter in correspondence with increments of traverse of the print head in the second direction and to initiate energisation of the printing elements to print the second imprint in response to the counter being stepped to a predetermined second count.